1. Field of the Invention
This invention relates to an improved burner and combustion method for use in the melting of glass, metals or other materials. More specifically, this invention relates to a burner and combustion method whereby hot, nearby, and substantially parallel fuel-rich and fuel-lean (oxidant-rich) jets efficiently transport fuel and oxidant to the surface of a material to be heated or melted by direct flame impingement. The method enables one to modify the flow so that either an oxidizing or reducing atmosphere is present at the surface.
2. Description of the Related Art
The heating and melting of materials such as glass cullet and batch, scrap metal, minerals and ores is important in many industries. Typically, each process has a heat input requirement, a heat distribution requirement and geometrical constraints which hinder the proper placement of a burner and often require the placement of a burner at a much greater distance from the material to be heated or melted than is optimal.
In addition, there are often chemical stability issues relative to heating materials. For example, direct flame impingement on iron or steel usually produces an undesirable oxide coating or oxide slag layer upon melting. Therefore, one would like to heat iron and steel with a reducing gas in contact with the solid or molten iron to minimize oxidation. Other examples of surfaces susceptible to oxidation or reduction include glass forming materials, aluminum, copper, metal alloys, lead, zinc, frit materials, or ceramic materials.
U.S. Pat. No. 5,139,558 to Lauwers discloses the use of oxy-fuel burners located on the furnace roof and aimed at the interface between the batch and the melt in order to increase the melting rate of the glass and to prevent batch materials from entering the upstream zone. However, because the furnace roof to glass melt distance is often greater than typical O2-fuel flame length, the thermal efficiency of the process is not adequate for use outside of regenerative or recuperative furnaces.
U.S. Pat. No. 5,643,348 to Shamp et. al attempts to overcome this problem by injecting oxygen and fuel at separate points and producing a large combustion xe2x80x9cflame cloudxe2x80x9d in the center of the furnace. This approach eliminates the oxy-fuel flame length limitation, but suffers from a safety problem. Where there is no apparent method to maintain reliable ignition of the fuel and oxidant this process could lead to explosions in applications where the ambient conditions are below the autoignition temperature. In addition, the cold fuel or oxidant jet could cause material to solidify and block the nozzle. With fuel-rich and fuel-lean flames of the present invention, the flame propagates back to the inlet nozzle, which keeps the nozzle clear and provides sufficient heat along the entire path of the fuel-rich or fuel-lean flame for ignition.
In U.S. Pat. No. 5,387,100 to Kobayashi the use of rich and lean fuel streams to reduce NOx emissions is disclosed, however, there is no teaching regarding the interacting of fuel-rich and fuel-lean flames or jets to increase the transfer of heat to the batch or melt. Kobayashi teaches forming fuel-rich and fuel-lean flames, allowing radiation to lower the flame temperature and then allowing the cooler flames to interact without contacting a surface to complete combustion at a lower temperature thereby decreasing NOx emission. In contrast, in the present invention, radiant losses are minimized prior to interaction with the surface to be heated.
U.S. Pat. No. 5,267,850 to Kobayashi et al. teaches the use of a fuel staged burner to keep the burner cooler. This invention uses similar equipment to produce a fuel rich jet adjacent to fuel lean jet or jets.
U.S. Pat. No. 5,100,313 to Anderson et al. teaches an approach to produce a hot oxygen jet. This oxygen jet is generally used to remove carbon from molten iron to produce steel.
Commonly assigned U.S. patent application Ser. No. 09/384,065 teaches the generation of fuel-rich and fuel-lean flames from separate burners. The flames then interact in gas space in the vicinity of a surface to more efficiently transfer heat to the surface.
The present invention teaches the generation of substantially parallel fuel-rich and fuel-lean jets from a single burner assembly to minimize energy loss during transport through the gas space. The fuel rich and fuel lean jets mix upon collision with a surface to produce a flame adjacent to the surface and provide a shield gas for the surface.
The ability to generate the fuel rich and fuel lean jets from a single burner assembly offers significant advantages over the prior art practice of introducing fuel rich and fuel lean flames from separate sources. Firstly, with separate sources the separate flames, the fuel rich and fuel lean jets, must intersect proximate to the surface to be heated. In the present invention the streams are substantially parallel thus relieving the fixed constraint of burner arrangement with burner to surface distance. This is of importance in situations where the surface of the material to be heated or melted varies. Secondly, the separate fuel rich and fuel lean streams need to intersect at distinct points, therefore, the burners need to be located at defined locations. These locations may be unavailable due to pre-existing structural constraints. The present invention removes the importance of these structural constraints. Thirdly, the alignment of multiple burners to generate intersecting flames at a defined point is difficult. The use of a single burner assembly removes that need.
Therefore, there is a clear and long-standing need for a burner and heating method that would maximize the amount of heat transferred to a surface of a material to be heated or melted in a variety of furnace geometries while allowing control of the reducing or oxidizing components present at the surface.
Accordingly, the present invention increases the amount of heat transferred to the surface of a material to be heated or melted in an industrial furnace by using a burner assembly having burner elements capable of producing fuel rich and fuel lean jets having shrouds of substantially stoichiometric combustion products and by contacting the surface with the fuel rich and fuel lean jets to form a flame at or near the surface.
The method can be used to heat the surface of materials susceptible to oxidation by generating a primary fuel stream as the core of the fuel rich jet, and generating an annular secondary oxygen stream around the primary fuel stream to generate the shroud of substantially stoichiometric combustion products at the periphery of the fuel rich jet. A plurality of primary oxygen streams around the periphery of the fuel rich jet each primary oxygen stream provides a core for each of the fuel lean jets, and an annular secondary fuel stream around each of the primary oxygen streams generates the shroud of substantially stoichiometric combustion products at the periphery of each of the fuel lean jets.
Alternatively, generating the fuel rich jet can be accomplished by generating a primary fuel stream as the core of the fuel rich jet and generating an annular secondary oxygen stream around the primary fuel stream to generate the shroud of substantially stoichiometric combustion products at the periphery of the fuel rich jet. The fuel lean jet is generated as an annular primary oxygen stream around the periphery of the fuel rich jet and an annular secondary fuel stream around the periphery of the primary oxygen stream to generate the shroud of substantially stoichiometric combustion products at the periphery of the fuel lean jet.
For use in heating the surface of a material susceptible to reduction, a primary oxygen stream is generated as the core of a fuel lean jet and an annular secondary fuel stream is generated around the primary oxygen stream to generate the shroud of substantially stoichiometric combustion products at the periphery of the fuel lean jet. A plurality of primary fuel streams are generated as the core of a plurality of fuel rich jets around the periphery of the fuel lean jet with an annular secondary oxygen stream generated around each of the primary fuel streams to create the shroud of substantially stoichiometric combustion products at the periphery of each of the fuel rich jets.
Alternatively, an annular primary fuel stream as the core of the fuel rich jet is generated around the periphery of the fuel lean jet with an annular secondary oxygen stream around the periphery of the annular primary fuel stream necessary to generate a shroud of substantially stoichiometric combustion products at the periphery of the fuel rich jet.
Diverting a portion of either or both of the primary oxygen stream or the primary fuel stream generates a lower velocity oxygen stream or fuel stream increasing the ability to generate a shroud of substantially stoichiometric combustion products at the periphery of the fuel rich and fuel lean jets.
The axis of the fuel lean jet or jets could diverge from the axis of the fuel rich jet by a divergence angle of not more than ten degrees. A cavity can also be used to focus the fuel rich and fuel lean jets. A portion of the cavity could diverge by not more than 20 degrees.
A further object of the invention is to operate the jets at an adiabatic temperature of the fuel rich and fuel lean jets of at least 800xc2x0 C. so as to insure combustion at or near the surface of the material to be heated or melted without the need for a supplemental ignition source.
A burner assembly for use in the method of the present invention includes a central conduit and a first annular conduit around the central conduit for producing a first fuel rich or fuel lean jet, said jet having a peripheral shroud of substantially stoichiometric combustion products. Either a plurality of second burner elements located around the periphery of the first burner element, each having a central conduit and an annular conduit, or an annular burner element located around the periphery of the first burner having a first annular and a second annular conduit produces the fuel lean or fuel rich jet having a peripheral shroud of substantially stoichiometric combustion products.
The utilization of fuel rich and lean jets limits radiant energy losses from the aforementioned streams, thus maximizing the heat delivered to the surface of the material to be heated. The generation of a shroud of combustion products at the periphery of the jets and a careful matching of velocities of the jets minimize the interaction of the rich and lean streams prior to their reaching the surface. The interaction of these jets, with the surface to be heated, completes contacting of the fuel-rich and fuel-lean jets to produce a high temperature flame adjacent to the surface. Intimate contact of a shroud gas and flame with surface achieves high heat transfer efficiency with substantial control of the gas properties in contact with the surface to be heated. The geometry of the hot fuel-rich and fuel-lean jets and their orientation relative to the surface is advantageously used to optimize the heat transfer rate and the oxidation potential of the gas in contact with the surface.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.